This invention relates to feed windows assembled to cover electronic components in a low noise block converter with integrated feed (LNBF) such as used in direct satellite broadcasting receivers and particularly relates to polyolefin based compositions suitable for such feed windows.
A low noise block converter (LNB) is used in communication satellite reception equipment and typically is mounted on or in a satellite antenna dish. In typical practice, communication satellites transmit signals using microwave range radio frequencies in the range of 10 to 40 gigahertz (GHz). Particularly useful for this use is the Ku band which ranges from about 12 to 18 GHz and more particularly the Ka band which ranges from about 18 to 40 GHz. In order to receive and use radiofrequency (rf) signals at an earth-based location, typically the microwave signals received from the communications satellite must be converted to lower or intermediate frequency signals at the point of reception. The lower frequency signals (typically in the range of 900 to 1500 MHz) then may be directed more easily and economically through cables to other locations. A low noise block converter is used to convert microwave range rf signals to intermediate rf signals. Typically, direct broadcast satellite (DBS) dishes use an LNBF which integrates the feedhorn of an antenna with an LNB. A typical DBS receiver is a parabolic dish with a feedhorn placed at the focal point of the dish.
In order to receive or transmit microwave rf signals from a DBS, an antenna typically is located outside of a building or structure and in line-of-sight to the satellite. Thus, the antenna with an LNB is subject to outside weather conditions including precipitation such as rain or snow. However, water is a very efficient absorber of microwave rf signals, and in order to minimize rf signal attenuation, water adhering on the antenna and especially on an LNB should be avoided. Thus, in usual practice the LNBF is covered with a feed window which is both hydrophobic and is substantially invisible to microwave signals. An example of an LNBF cover is described in U.S. Pat. No. 6,072,440.
Use of HDPE and ABS thermoplastic polymers for antenna cover assemblies have been described in U.S. Pat. No. 6,191,753 as weather resistant. Laminates have been used as described in U.S. Pat. No. 5,815,125 as covers using a porous polytetrafluoroethylene outer layer laminated to a thermoplastic substrate, although usually, laminate materials are costly to manufacture. Alternatives to laminates include external non-stick or hydrophobic coatings such as described in U.S. Pat. No. 4,536,765. However, external coatings may wear through weathering or abrasion and provide a diminished hydrophobic surface over time. Complex protective shields for high performance antennal arrays have been described in U.S. Pat. No. 4,783,666 as a sandwich formed between fiberglass layers and a central foam core on which a polytetrafluoroethylene layer is applied coated with fumed silica (SiO2). The polytetrafluoroethylene-fumed SiO2 was said to minimize effects of rain on antenna performance.
A feed window or LNBF cover may be formed from a polyolefin thermoplastic such as a propylene polymer. Although polypropylene has some hydrophobic properties, the hydrophobic character of polypropylene alone typically is insufficient for current applications.
We have discovered that a feed window formed from a polyolefin composition containing a specified amount of a high molecular weight siloxane, especially a polydialkylsiloxane, shows a substantial increase of hydrophobic character over the polyolefin alone. Such a uniform composition is easily mouldable, does not degrade through weathering and abrasion, and is economical.